1. Field of the Invention
This invention relates to a low-temperature fired ceramic circuit substrate formed by co-firing a low-temperature firable ceramic green sheet with Ag wiring conductors, and to a thick-film paste used in the fabrication thereof.
2. Description of the Prior Art
An Ag wiring conductor has good electric characteristics such as a low sheet resistance. However, since the Ag wiring conductor has a melting point lower than a firing temperature (about 1,600.degree. C.) of general ceramic substrates such as an alumina substrate, the Ag wiring conductor cannot be used for the alumina substrate. Tungsten (W) or molybdenum (Mo) each having a higher melting point has been used as the wiring conductors for the alumina substrate. However, these metals with high melting points have high sheet resistances and need to be fired at high temperatures in a reducing atmosphere in order to inhibit oxidation.
To overcome the above-described drawbacks, U.S. Pat. No. 4,621,066 discloses a low-temperature firing ceramic substrate which can be fired in an oxidizing atmosphere at a temperature between 800.degree. and 1,000.degree. C., i.e., at or below the melting point of the Ag wiring conductor. The Ag wiring conductor is co-fired with the low-temperature ceramic circuit substrate. The Ag wiring conductor causes migration under specific conditions, whereas electrode portions and the like on the surface of the substrate require migrationproofing. Accordingly, a film of Au conductor superior in migrationproofing needs to be formed on the Ag conductor of the electrode portions. However, when the Ag conductor is fired with the Au conductor being connected directly thereto, the Kirkendall effect causes Ag atoms to diffuse into the Au conductor, which results in a number of voids formed in the connected surface. Consequently, the reliability in the connection is reduced.
To prevent the above-described problem, Japanese Patent Publication No. 5(1993)-69319 discloses an intermediate layer of metal such as Ni, Cr, or Ti. The intermediate metal layer is formed between the Ag and Au conductors by means of metal plating, sputtering, or the like for preventing diffusion of the Ag atoms into the Au conductor so that the reliability in the connected portion is enhanced.
However, the metal plating or sputtering employed for forming the intermediate metal layer is costly when the same is provided between the Ag and Au conductors, whereupon the cost of the ceramic substrate is increased. It has been considered that a thick-film printing technique may be employed for provision of the intermediate metal layer. However, nickel needs to be fired in an atmosphere of nitrogen for the inhibition of oxidation, which also increases the cost of the ceramic substrate.
Additionally, the substrate is repeatedly fired at a temperature between 800.degree. and 900.degree. C. for the forming of other circuit elements such as resistance after the thick film of the Au conductor has been formed on the intermediate metal layer. The repeated firing sometimes results in breakage or disconnection in a connection between the Ag and Au conductors. Consequently, an enhancement in the reliability of the connection between the Ag and Au conductors has been desired in the ceramic substrate together with reduction in the cost thereof.